In addition to the qualitative difference in x-rays from two different machines using the same filtration and having the same half value layer (HVL) and the dose dependent directional effects of the exposure, other physical factors affect the biological effects of x-irradiation. Dose rate accounts for the difference between dorsal and two-directional exposure but not the differences between alternate dorsal + ventral and ventral + dorsal exposures compared with dorsal and ventral exposures. Dose dependent directional effect also occur following exposure to the higher energy gamma radiation from Cs-137. Conclusions: a) the HVL should no longer be considered sufficient indication of the quality of x-ray; b) 250 kV x-ray machines should no longer be used as a standard for calculating relative biological effectiveness (RBE) of other radiation; c) data on males and females should not be combined without first demonstrating that significant dose dependent sex differences do not occur; d) the mean tissue dose obtained with a one directional lateral Cs-137 source should not be expected to produce the same biological effect as the uniform tissue dose produced by a two directional (dorsal-ventral) Cs-137 irradiator.